Why are unicelluar orginasims ussally small

The need for a large surface area through which to absorb nutrients from the environment is a major restriction on the size of a cell, and it's why single-celled organisms can only grow so large before they have to work together or divide into multicellular organisms.

Explanation:

Hope this helps :)

step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.